The long-term objective of the proposal is to elucidate the mechanisms of exosome biogenesis and to explore the scope and limitation of the relationship between exosomes and retroviruses. Exosomes are small, secreted organelles with the same topology and similar size as retrovirus particles and it has been proposed that retroviruses such as HIV are "Trojan exosomes". Recent studies provide support for this hypothesis and warrant more detailed studies of how exosomes are made. Using a wide variety of animal cell culture systems and molecular genetic and biochemical techniques we will (aim 1) determine the scope and limitation of the relationship between exosomes and retroviruses such as HIV, (aim 2) identify the signals that target exosomal proteins to sites of exosome budding and into exosomes, (aim 3) identify and characterize the genes and proteins that mediate exosome biogenesis, and (aim 4) explore the additional roles of exosome biogenesis pathways in the cell.